Japanese Utility Model Publication No. 7939/1974 (JP-Y-49-7939) discloses a flexible synthetic resin tube which is constituted with an inner layer of a thermoplastic polyurethane resin and an outer layer of a plasticized or soft vinyl chloride resin, the both layers being thermally fused and bonded together to improve the abrasion-resistant property of the tube. Disclosed in one of the embodiments is a flexible tube which has a spiral core of a non-plasticized or hard poly(vinyl chloride) resin embedded or buried in the boundary area between the inner layer and the outer layer.
However, this literature fails to disclose the details of the material for forming the inner layer of the flexible synthetic resin tube, simply referring to a thermoplastic polyurethane resin as such. If the polyurethane resin is a polyester-series thermoplastic polyurethane resin, such selection utterly falling to consider hydrolysis-resistance of the resin, the thermoplastic polyurethane resin is hydrolysed after exposed to a high temperature and high humidity for a long time. As a result, physical properties of the tube, such as mechanical strength, flexibility, and elasticity, are likely to deteriorate.
Suppose that the material is a polyether-series thermoplastic polyurethane resin, it is practically impossible to fuse this resin thermally directly with a soft vinyl chloride resin to unite these resins, and to apply the resulting synthetic resin tube to various uses. For this reason or others, this literature proposes to use, as the soft poly(vinyl chloride) resin to constitute the outer layer, a soft poly(vinyl chloride) resin modified with a specific plasticizer or a carboxyl group-containing copolymer of vinyl acetate and vinyl chloride, in order to improve thermofusability or thermal-fusion property to the polyurethane resin.